1. Field of the Invention
The invention relates to multiple-ratio automatic transmissions for automotive vehicles wherein ratio changes between ratios involves engagement and disengagement of friction elements that establish and disestablish torque flow paths from a torque input member to a torque output member.
2. Background Art
The control of gear elements of multiple-ratio gearing for an automotive vehicle transmission are controlled typically by friction clutches or brakes, hereinafter referred to as torque transfer friction elements, to establish and to disestablish each of several forward driving ratios and one or more reverse ratios. Examples of automotive transmissions of this type are shown in U.S. Pat. Nos. 6,292,731; 5,722,519; 5,553,694; 5,758,302; 6,370,463; and 6,577,939. These transmissions include controls for establishing and disestablishing ratio changes using both nonsynchronous and synchronous engagement and release of the friction elements. An example of a transmission that uses only synchronous ratio changes is disclosed in co-pending U.S. patent application Ser. No. 10/712,071, filed Nov. 13, 2003, by Steve Cicala et al., which is entitled “Adaptive Pressure Control Method For Achieving Synchronous Upshifts In A Multiple-Ratio Transmission”, now U.S. Pat. No. 6,994,647. That co-pending patent application is assigned to the assignee of the present invention.
In a transmission characterized by synchronous ratio upshifts and downshifts, the ratio change requires engagement of one friction element as a companion friction element is disengaged. This engagement and disengagement must occur in synchronism. Precise synchronization is required to achieve acceptable shift quality. Precision in the synchronization must be maintained, furthermore, throughout the life of the transmission, notwithstanding the presence of wear of the torque transfer friction elements and changes in environmental conditions, such as temperature changes, lubricant viscosity changes, and changes in coefficients of friction for the friction elements.
Errors in synchronization during ratio changes cause perceptible inertia torque disturbances at a transmission torque output shaft that have an adverse effect on overall shift quality. Shift quality would be affected, for example, if the ratio change were to start as pressure on the oncoming friction element is increased at the start of the event during a synchronous downshift. Other errors in synchronization during a synchronous downshift are slip time errors for the offgoing friction element as the oncoming friction element gains capacity.